A numeric identifier of every member in the replica set. Once
set, you cannot change the
_id of a member.

Note

When updating the replica configuration object, access the replica set
members in the members array with the
array index. The array index begins with 0. Do not confuse
this index value with the value of the
_id field in each document in
the members array.

A boolean that indicates whether the mongod builds
indexes on this member. You can only set this
value when adding a member to a replica set. You cannot change
buildIndexes field after
the member has been added to the set. To add a member, see
rs.add() and rs.reconfig().

Do not set to false for mongod instances that receive
queries from clients.

Setting buildIndexes to false may be useful if all
the following conditions are true:

you are only using this instance to perform backups using
mongodump, and

this member will receive no queries, and

index creation and maintenance overburdens the host
system.

Even if set to false, secondaries will build indexes on the
_id field in order to facilitate operations required for
replication.

When this value is true, the replica set hides this instance
and does not include the member in the output of
db.isMaster() or isMaster. This prevents
read operations (i.e. queries) from ever reaching this host by
way of secondary read preference.

A number that indicates the relative eligibility of a member to
become a primary.

Specify higher values to make a member more eligible to become
primary, and lower values to make the member less
eligible. Priorities are only used in comparison to each other.
Members of the set will veto election requests from members when
another eligible member has a higher priority value. Changing the
balance of priority in a replica set will trigger an election.